Optically addressed spatial light modulating system and method for driving the system

ABSTRACT

An optically addressed spatial light modulation system includes a ferroelectric liquid crystal spatial light modulator. A writing light source irradiates a writing light for recording an image onto the spatial light modulator. A read-out light source irradiates a bias light for adjusting the sensitivity of the spatial light modulator and a read-out light for reading a recorded image from the spatial light modulator. An adjusting circuit is used to adjust the bias light intensity or irradiation time in synchronism with the writing light to increase the sensitivity of the spatial light modulator. A driving circuit supplies writing voltage signals to the spatial light modulator. The irradiation times of the write light and the bias light overlap with the application of the writing voltage signals for a predetermined time for adjusting the sensitivity of the spatial light modulator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 07/954,316,filed on Sep. 30, 1992 now U.S. Pat. No. 5,555,115.

BACKGROUND OF THE INVENTION

This invention is related to an optically addressed spatial lightmodulator and to a method for changing the sensitivity of an opticallyaddressed spatial light modulator using a ferroelectric liquid crystalmaterial as a light modulation material in the fields of opticalinformation processing and optical measurement.

In the fields of optical information processing and optical measurement,as research has progressed, there arises a need for a spatial lightmodulator having high resolution and fast response time. Conventionally,an electro-optic crystal material such as a BSO crystal material (Bi₁₂SiO₂₀ crystal) has been used as a light modulation material in manyoptical addressing spatial light modulators, and a liquid crystal lightvalve using a nematic liquid crystal material has also been employed asan optical addressing spatial light modulator in many cases. However,these materials are not sufficient to meet the above requirementsrelating to resolution and response time. There has recently beendeveloped and already used an optical addressing spatial light modulatorusing a ferroelectric liquid crystal material as the light modulationmaterial (hereinafter abbreviated as "FLC-OASLM").

First of all, the structure of the FLC-OASLM will be explained. TheFLC-OASLM is different from the conventional liquid crystal light valveusing nematic liquid crystal material in respect of using, as a liquidcrystal layer, ferroelectric liquid crystal material which has clearbistability between light transmittance or light reflectance and anapplied voltage. FIG. 2 is a sectional view showing the structure of theFLC-OASLM. On the surfaces of transparent substrates 101a, 101b made ofglass, plastic or the like for sandwiching liquid crystal molecules areprovided transparent electrode layers 102a, 102b, and alignment layers103a, 103b which are formed by evaporating silicon monoxide obliquely atan angle ranging from 75° to 85° with a normal direction of therespective transparent substrates. The transparent substrates 101a, 101bmake the respective alignment layers 103a, 103b to be opposed bycontrolling a gap through a spacer 109 and to sandwich a ferroelectricliquid crystal layer. There are laminated a photoconductive layer 105, alight blocking layer 106, and dielectric mirror 107 on the transparentelectrode 102a of an optical write side of the FLC-OASLM and under thealignment layer 103a. Anti-reflection coating layers 108a, 108b areformed respectively on the outsides of the transparent substrate 101a onthe write side and the transparent substrate 101b on the read side,which constitute a cell.

Next, two methods for initializing the FLC-OASLM having the abovestructure are described. In the first method, an entire plane of thewrite side of the FLC-OASLM is irradiated with light. A pulse voltage, adirect current bias voltage, or a direct current bias voltage which issuperimposed with an alternating current voltage between 100 Hz and 50kHz is applied as an erasing voltage between the transparent electrodelayers 102a and 102b. These voltages are sufficiently higher than athreshold voltage at the time of irradiation. As a result, all of theferroelectric liquid crystal molecules are arranged in one directionresulting in a stable status, and the status is recorded. In the secondmethod, the FLC-OASLM is not irradiated with light at all. A pulsevoltage, a direct current bias voltage, or a direct current bias voltagewhich is superimposed with an alternating current voltage between 100 Hzand 50 kHz is applied as an erasing voltage between the transparentelectrodes 102a and 102b. These voltages are sufficiently higher than athreshold voltage at the time of no irradiation. Then, all offerroelectric liquid crystal molecules are arranged in one directionresulting in a stable status, and the status is recorded. Generally, thethreshold voltage at the time of no irradiation is higher than that atthe time of irradiation.

Further, explanation will be given as to operations to be performedafter the FLC-OASLM is initialized as explained above. A pulse voltage,a direct current bias voltage, or a direct current bias voltage which issuperimposed with an alternating current voltage between 100 Hz and 50kHz is applied as a write voltage between the transparent electrodelayers 102a and 102b. The write voltage has a polarity reverse to thatof the voltage used for initialization, and is lower than the thresholdvoltage when light is irradiated, and higher than the threshold voltagewhen no light is irradiated. While the write voltage is applied, animage is optically written by laser beam or the like. Carriers aregenerated in the photoconductive layer 105 in the region irradiated withlaser, and the carriers move toward an electric field. As a result, thethreshold voltage declines, and an applied voltage which is higher thanthe threshold voltage and has a polarity reverse to that of the voltageused for initialization is applied to the region irradiated with laser.Then, the molecules reverse in the ferroelectric liquid crystal materialaccompanying the reverse of spontaneous polarization, and theferroelectric liquid crystal material turns from one stable status toanother. Therefore, an image is binarized and recorded. The recordedimage remains recorded even when the drive voltage becomes zero.

The image binarized and recorded in the above manner can be read outeither in a positive or a negative manner by irradiating the device witha read light of linearly polarized light which is arranged so that itspolarization axis should be in the direction of the liquid crystalmolecules arranged in one direction by initialization (or in a directionperpendicular to the above direction), or by passing the light reflectedby a dielectric mirror 107 through an analyzer which is arranged so thatits polarization axis should be perpendicular to (or parallel to) thepolarization direction of the reflected light. A polarization beamsplitter is often used as an analyzer.

In theory, it is possible to initialize the FLC-OASLM and memorizes animage in the above-mentioned method. However, as a practical method fordriving the FLC-OASLM, such driving voltage as indicated in FIG. 3 isapplied to the FLC-OASLM in order to record, erase and read out an imagein many cases. FIG. 3 shows one example of driving voltage wave formswhich are applied to the FLC-OASLM when the transparent electrode layer102a on the read side is grounded. In the conventional FLC-OASLM, writelight and read light always irradiate respectively the write side (theside of the transparent substrate 101a) and the read side (the side ofthe transparent substrate 101b) of the FLC-OASLM. The FLC-OASLM isinitialized by being applied with a positive pulse voltage (referred toas an erasing pulse 31) which is an erasing voltage. A picture image isrecorded by a negative pulse voltage (referred to as a write pulse 32)as a write voltage, and an image memorized by the write pulse 32 andzero voltage 33 is read out. With this method, the FLC-OASLM can bedriven in the frequency range from tens of Hz to several kHz.

Actually, however, it is still difficult to fabricate an FLC-OASLM whichincludes the dielectric mirror 107 or the light blocking layer 106 as alight reflecting and separating layer and is uniform in a large area,and the FLC-OASLM which does not include the light reflecting andseparating layer is often used. One of the causes of this is thatinstalling the light blocking layer 106 or the dielectric mirror 107makes it difficult to control a gap of about 1 to 2 μm for injectingferroelectric liquid crystal material and to control the alignment ofthe ferroelectric liquid crystal material. In the FLC-OASLM having thisstructure, a read out light is reflected on an interface of thephotoconductive layer 105. The reflectance of the read-out light isapproximately 20% if hydrogenated amorphous silicon is used as thephotoconductive layer 105 and the wavelength of the read-out light is633 nm.

As the photoconductive layer 105, single crystal BSO (Bi₁₂ SiO₂₀) orsingle crystal silicon is sometimes used, but hydrogenated amorphoussilicon is used in many more cases at the present stage. The reason forusing hydrogenated amorphous silicon is that the response time as aspatial light modulator can be shortened, resolution can be improved,the thickness of the photoconductive layer 105 can be reduced, andmanufacture of devices using it is relatively easy.

However, a light addressed spatial modulator, especially the FLC-OASLMdescribed above, has some problems as follows. When hydrogenatedamorphous silicon is used as the photoconductive layer 105, forinstance, the energy necessary for writing to the FLC-OASLM per minimumspot area is in the range from 0.03 pJ to 0.2 pJ and does not have amultiplication action as does a photomultiplier. It cannot be said thatsuch recording sensitivity is adequate. In other words, the major factorin restricting the application of the FLC-OASLM is that an image cannotbe recorded without the write light of fairly strong intensity.

When the FLC-OASLM is used as an incoherent to coherent transducer, asone example of the application, outdoor scenery, parts flowing alongproduction lines in a factory and so on are imaged on thephotoconductive layer 105 by the use of an imaging lens in order torecord images of the scenery and the parts. However, the intensity ofsuch a write light (intensity of the image projected on the FLC-OASLM)is generally much too weak to be recorded. Further, if a Fouriertransformed image is recorded on the FLC-OASLM used in an opticalpattern recognition system and the like, a high frequency component ofthe Fourier transformed image has a remarkably weaker intensity ascompared with its low frequency component. Consequently, the highfrequency component cannot be recorded, and accurate pattern recognitionis difficult.

In the above case, images could be recorded by improving the sensitivityof the FLC-OASLM itself or by increasing the intensity per unit area ofthe write light. Then, the write light intensity per unit area could beintensified by raising the luminance of the light source illuminating anobject or by reducing the size of a written image. However, it is verydifficult to improve substantially the sensitivity of the FLC-OASLMitself and the luminance of the light source. Further, the size of thewritten image seldom can be substantially reduced because of problemsrelated to the resolution and application of the FLC-OASLM. As explainedabove, in the conventional method the application of the FLC-OASLMcannot be expanded, for example, by application to the incoherent tocoherent transducer, because it is difficult to increase the intensityof the written image for the purpose of writing an image with a weakwrite light intensity to the FLC-OASLM.

There is another problem in that the sensitivity is different dependingon the wavelength of the write light, which is due to spectralsensitivity characteristics. For instance, if hydrogenated amorphoussilicon is used as the photoconductive layer 105, sensitivity is high atwavelengths approximately between 600 nm and 650 nm, but quite low atother wavelengths. As a result, when white light is used as the lightsource of the write light, an image cannot be recorded even if theoverall light intensity is strong but the spectral intensity of thewavelength at which sensitivity is high for hydrogenated amorphoussilicon is weak. Moreover, there could be a case in which a wavelengthwith low sensitivity must be used as the write light for someapplications. In such a case, recording of an image is also difficult.

Further, it is known that the accuracy of pattern recognition greatlychanges if the threshold for recording the written image is changed,such as when an image including noise or a Fourier transformed image isrecorded on the FLC-OASLM which is used in an optical patternrecognition system. It is easy to make the threshold higher, forexample, to eliminate noise or unwanted sidebands, because it isequivalent to reducing the write light intensity with an ND filter andthe like. However, there is a drawback in that it is very difficult, onthe other hand, to record the written image by lowering the thresholdbecause the sensitivity of the FLC-OASLM is insufficient.

The above drawback is not limited to the FLC-OASLM, but similardrawbacks exist as to general optically addressed spatial lightmodulators.

SUMMARY OF THE INVENTION

In view of the above drawbacks of conventional optically addressedspatial light modulators, an object of the present invention is to makeit possible to change recording sensitivity easily in an opticallyaddressed spatial light modulator which uses a liquid crystal material,for example a ferroelectric liquid crystal material, as a spatial lightmodulator material. The present inventive optically addressed spatiallight modulator is composed of write light irradiation means forirradiating a write light used to write an image to the spatial lightmodulator; read out light irradiation means for irradiating a read outlight used to read out the recorded image to the spatial lightmodulator; bias light irradiation means for irradiating a bias lightwhich is used to adjust a recording sensitivity of the spatial lightmodulator; and bias light adjustment means for changing at least eitherirradiation time and the light intensity of the bias light.

Further, when the spatial light modulator uses a ferroelectric liquidcrystal material as an optical modulation material, the spatial lightmodulator is provided with driving means for driving the spatial lightmodulator, and the irradiation time of the write light and theirradiation time of the bias light overlap respectively with the writevoltage application time of the spatial light modulator at least for aspecific time.

In the method for driving the optically addressed spatial lightmodulator which is structured as mentioned above, especially in case ofthe FLC-OASLM, light from an auxiliary light source or the read outlight itself is irradiated to the FLC-OASLM as the bias light for thewrite voltage application time of the FLC-OASLM or for a part of thattime. As the write light and the bias light have a similar action on aphotoconductive layer of the FLC-OASLM, an image can be binarized andrecorded when the sum of the light intensity of the write light and thebias light exceeds a threshold of the FLC-OASLM. In other words, athreshold of the write light can be change relatively in proportion tothe intensity of the bias light by using the bias light. It is also easyto control the threshold of the write light by adjusting the intensityof the bias light or irradiation time of the bias light. As a result, itbecomes possible to record an image, which could not heretofore berecorded because its write light intensity per unit area is too weak, onthe FLC-OASLM which is driven at a high speed, or to record a writtenimage the threshold of which is varied. This improvement is common tooptically addressed spatial light modulators which use other materialsthan the ferroelectric liquid crystal material as the light modulationmaterial. In that case, it is not the threshold for binarizing an imageto be recorded but the sensitivity that can be recorded.

Other features and advantages of the present invention will be apparentto those skilled in the art from the following description of thepreferred embodiments, taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating one example of the concept of themethod for driving the inventive optically addressed spatial lightmodulator.

FIG. 2 is a sectional view showing the structure of the FLC-OASLM of theprior art.

FIG. 3 is a waveform chart showing one example of the driving voltagewhich is to the FLC-OASLM, in the prior art.

FIG. 4 is an explanatory view showing a change in sensitivity of theinventive optically addressed spatial light modulator.

FIG. 5A is a structure view showing a structure of one embodiment of themethod for adjusting the sensitivity of the inventive opticallyaddressed spatial light modulator.

FIG. 5B is another structural view showing the structure of anotherembodiment of the method for adjusting the sensitivity of the inventiveoptically-addressed spatial light modulator.

FIGS. 6(a)-6(c) are waveform charts showing a driving method of oneembodiment of the method for adjusting the sensitivity of the inventiveoptically addressed spatial light modulator.

FIGS. 7(a)-7(c) illustrate a driving method of another embodiment of themethod for adjusting the sensitivity of the inventive opticallyaddressed spatial light modulator.

FIGS. 8(a)-8(c) are waveform charts illustrating a driving method ofanother embodiment of the method for adjusting the sensitivity of theinventive optically addressed spatial light modulator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be explained with reference tothe attached drawings.

FIG. 1 illustrates the concept of a method for adjusting the sensitivityof the inventive optically addressed spatial light modulator.Explanation will be given on the supposition that a FLC-OASLM, having alight reflecting and separating layer explained in FIG. 2, is taken asan optically addressed spatial light modulator. The FLC-OASLM 1 isdriven by a driving means 41, and write light 4 is irradiated to itswrite side 2 by write light irradiation means 42, and read out light 5is irradiated to its read side 3 by read out light irradiation means 43.Further, bias light 6 is irradiated to the write side 2 by bias lightirradiation means 44. The irradiation time of the bias light 6 and itsintensity are adjusted by bias light adjustment means 45. It is needlessto say that the bias light 6 may irradiate not the write side 2 but theread side 3 or both of the sides, if the FLC-OASLM 1 does not have thelight reflecting and separating layer, or if the bias light irradiatingthe read side affects a photoconductive layer 105 because of a fairlystrong intensity of the bias light, or for other reasons even when theFLC-OASLM 1 does have the light reflecting and separating layer.

FIG. 4 is an explanatory chart showing a change in the sensitivity ofthe FLC OASLM. In FIG. 4, a solid line represents the case where thebias light is not irradiated and a broken line represents the case wherethe bias light is irradiated. As the FLC-OASLM 1 uses a ferroelectricliquid crystal material as a light modulation material, its recordingcharacteristics generally have a clear threshold. Then, the lightintensity of an image which is read out as a positive image withintensity distribution is in a dark state when a write light intensityis Ith or less, and is in a bright state when the write light intensityexceeds Ith. The bias light 6 is irradiated to the write side 2 of theFLC-OASLM 1 having such a characteristic. In order to simplifydiscussion, it is supposed that though the write light 4 and the biaslight 6 have the same wavelength, they do not interfere with each otherbecause of low coherence of the two lights, and that the light intensityon the write side 2 is equivalent to a sum of the light intensity of thewrite intensity 4 and the bias light 6. This feature is basically thesame when the coherence is high. However, as an interference fringe isformed because of an interference between the write light 4 and the biaslight 6, it is necessary to come up with some method to make a width ofthe interference fringe small enough as compared with resolution of theFLC-OASLM 1. The write light 4 and the bias light 6 irradiate the writeside 2 for the same period of time. Further, as it is supposed that theFLC-OASLM 1 has the light reflecting and separating layer, any influenceof the read light 5 is neglected. When the intensity of the bias light 6on the write side is IB, an image can be recorded if the sum of theintensity IB of the bias light 6 and intensity IW of the write light 4,IB+IW, is the threshold Ith of the FLC-OASLM 1 or more. As a result, thethreshold IthW of the write light 4 becomes Ith-IB and changes only by-IB even if the threshold Ith itself of the FLC-OASLM 1 is constant, sothat it is possible to change relatively the threshold of IthW of thewrite light 4. Therefore, the threshold of IthW of the write light 4 canbe easily controlled by changing the intensity of the bias light 6.

Further, when the bias light 6 on the photoconductive layer 105 of theFLC-OASLM 1 has light intensity distribution, the threshold IthW of thewrite light 4 can be distributed within the plane. For instance, when aFourier transformed image is used as the write light 4, the FLC-OASLM 1is irradiated with the bias light 6 and the intensity becomes strongeraccording to the degree the bias light 6 is separated from an opticalaxis of the write light 4. In such a case, the more the bias light 6 isseparated from the optical axis, the lower the threshold IthW of thewrite light becomes, and consequently high frequency elements of theFourier transformed image can be recorded. However, as it is generallydesirable that the threshold IthW should be uniform within the plane,the bias light 6 has a uniform light intensity distribution so as toirradiate the photoconductive layer 105 uniformly.

Additionally, the irradiation time of the bias light 6 is also used forthis purpose because the threshold Ith of the write light 4 depends onnot only the intensity but also the irradiation time. That is, thethreshold IthW of the write light 4 can be changed by adjusting the timeduring which the write voltage application and the irradiation of thebias light 6 overlap and the intensity IB of the bias light 6. It isunderstood that the irradiation time of the write light 4 and the writevoltage application time have to coincide for at least a fixed time inorder to record a written image. However, the irradiation time of thewrite light 4 and the irradiation time of the bias light 6 do not haveto overlap within the write voltage application time.

The above explanation refers to a case of recording the written imageinto the FLC-OASLM 1 by way of binarization. It is understood, however,that sensitivity can be changed in a similar manner by changing awaveform of the driving voltage applied to the FLC-OASLM 1 when thewritten image is recorded as an image having a gray scale. It is alsounderstood that this method can be used not only for the FLC-OASLM 1 butalso for any other optically addressed spatial light modulators,1whether of a transmittable type or of a reflective type, or even if anyof a BSO crystal material, a lithium niobate crystal material (LiNbO₃),and so on is used as the light modulation material, or even if it doesnot show bistability of the sensitivity. In case of a spatial lightmodulator which does not show the bistability of the optical response,it is not a threshold for binary recording but the sensitivity that canbe controlled. However, it is needless to say that the driving means 41is unnecessary when the driving voltage from outside is not required torecord or erase an image such as lithium niobate crystal.

FIG. 5A is a structure diagram showing a structure of one embodiment ofa method for adjusting the sensitivity of the inventive opticallyaddressed spatial light modulator. Explanation will be given here bysupposing the FLC-OASLM 1 is one which does not have the lightreflecting and separating layer. A means for driving the spatial lightmodulator is a FLC-OASLM driving circuit 19. A means for irradiating thewrite light used to write an image into the spatial light modulatorincludes a CRT 11 and an image lens 12. A means for irradiating the readout light used to read out an image which is recorded in the spatiallight modulator includes a read out laser 16, a power source 21 of theread laser, a beam expander 17, and a polarization beam splitter 18. Ameans for irradiating the bias light to the spatial light modulator iscomposed of an auxiliary light source 13, a diffusion plate 14, aprojection lens 15, and a power source 20 of the auxiliary light source.A bias light adjustment means for changing at least either irradiationtime of the bias light or intensity of the bias light includes asynchronous circuit 22.

An image which is displayed as the write light on the CRT 11 is imagedand irradiated on the photoconductive layer of the FLC-OASLM 1 by theimaging lens 12. Light generated from the auxiliary light source 13 isirradiated to the diffusion plate 14 for eliminating an intensitydistribution of the light source itself and is irradiated to the writeside 2 as the bias light 6 by the projection lens 15. On the other hand,on the side of the read light, a beam outgoing from the read out laser16 is expanded by the beam expander 17 and then reflected by thepolarization beam splitter 18 to irradiate the read side 3 of theFLC-OASLM 1 as the read out light 5. The image recorded on the FLC-OASLM1 is read out as a positive image or a negative image with an intensitydistribution caused by the effect that the read light 5 reflected on theFLC-OASLM 1 is transmitted through the polarization beam splitter 18. Atthis stage, the FLC-OASLM driving circuit 19, the power source 20 of theauxiliary light source, and the power source 21 of the read laserrespectively drive the FLC-OASLM 1, the auxiliary power source 13, andthe read out laser 16 while they synchronize by synchronization signalsfrom the synchronous circuit 22.

FIG. 5B is structural diagram showing another embodiment of a method foradjusting the sensitivity of the inventive optically addressed spatiallight modulator. In this embodiment, the reading out laser 16 and powersource 21 are used as means for irradiating a bias light to the spatiallight modulator. As in the previous embodiment, the bias lightadjustment means includes the synchronous circuit 22.

FIGS. 6(a)-6(c) illustrate a driving method used in one embodiment of amethod for adjusting the sensitivity of the inventive opticallyaddressed spatial light modulator. FIG. 6(a) shows a driving voltagewaveform of the FLC-OASLM 1 at the time of grounding a transparentelectrode layer 102a on the read side. FIG. 6(b) shows a change inintensity of the read out light. FIG. 6(c) shows a change in intensityof the image which is read out from the FLC-OASLM 1. As shown in FIG.6(a), the FLC-OASLM 1 is applied with a driving voltage in which anerase pulse 31, a write pulse 32 and a zero voltage 33 are alternatelyrepeated. In order to explain briefly, suppose that the write light 4and the bias light 6 constantly irradiate on the write side of theFLC-OASLM 1. Since the FLC-OASLM 1 is assumed not to have the lightreflecting and separating layer, the read out light 5 is cut off onlywhile the write pulse 32 is applied during the write voltage applicationtime, as shown in FIG. 6(b). The read out light 5 is modulated bymodulating directly an outgoing light of the read out laser 16 such as alaser diode with the synchronous circuit 22 and the power source 21 ofthe read laser. This prevents the read light 5 from affecting thephotoconductive layer 105 at the time of writing.

In the FLC-OASLM 1 driven in the above-mentioned manner, the write light4 and the bias light 6 are irradiated while the write pulse 32 isapplied. Consequently, the threshold of the FLC-OASLM 1 relativelydeclines, and an image included in the write light is given a thresholdat a lower value and recorded. Then, the threshold of the write light 4can be controlled by changing the intensity of the bias light 6. If anLED (light emitting diode), for instance, is used as the auxiliary lightsource 13, the brightness of the LED can be easily changed when acurrent applied to the LED changes. Therefore, the intensity of the biaslight 6 can be changed by the current applied to the LED, so that thethreshold of the write light in the FLC-OASLM 1 can be controlled by thecurrent applied to the LED.

Assuming that the bias light 6 does not constantly irradiate the writeside 2 of the FLC-OASLM 1 but that its light intensity is modulated,whether the intensity of the write light 4 is modulated or not, the sameeffect can be obtained as the intensity of the bias light 6 is changed,if the time to irradiate the bias light 6 during the write voltageapplication time while the pulse 32 is applied to the FLC-OASLM 1 ischanged. This method can also be easily performed by use of the LED asthe auxiliary light source 13. Since the written image is read out bythe read out light 5 and erased by the erase pulse 31, the lightintensity of the image read out as a positive image turns to a darkstate when the erase pulse 31 is applied, as shown in FIG. 6(c).

As the auxiliary light source 13, a red LED for the monochromatic lightand a common miniature bulb and so on for the white light are available.It is understood that the sensitivity can be changed by any auxiliarylight source 13 which can generate a light having an adequate wavelengthto generate carriers on the photoconductive layer 105 of the FLC-OASLM1, whether it is a monochromatic light or a white light, though thesensitivity of each depends on the wavelength.

Further, in order to irradiate the bias light 6, it is irradiated on thewrite side 2 of the FLC-OASLM 1. However, it is obvious that thesensitivity can be changed even if the bias light 6 irradiates the readside 3 of the FLC-OASLM 1, when the FLC-OASLM 1 does not have the lightreflecting and separating layer, such as a dielectric mirror 107 or alight blocking layer 106, or when light from the read side 3 affects thephotoconductive layer 105 and the FLC-OASLM 1 has the light reflectingand separating layer.

It is understood that the sensitivity of the FLC-OASLM 1 can be changesdepending on mask patterns and is not uniform if a liquid crystaltelevision set is provided as a mask in an optical path between thediffusion plate 14 and the FLC-OASLM 1.

In the above embodiment, outgoing light from the read out laser 16 suchas a laser diode is directly modified by the power source 22 of the readlaser in order to modify the intensity of the read out light. However,it is evident that outgoing laser light can be used as the read outlight 5 by modulating its intensity by a liquid crystal shutter whichuses a nematic liquid crystal material or a ferroelectric liquid crystalmaterial as the light modulation material, when He-Ne laser and so onare used as the read out laser 16. It is needless to say that theintensity of the read light 5 does not need be modulated, if theFLC-OASLM 1 has the light reflecting and separating layer or if theintensity of the read out light 5 is weak and the read out light 5 doesnot affect the photoconductive layer 105.

As the write light 4, the above embodiment uses an image which isdisplayed on the CRT 11 and imaged on the FLC-OASLM 1, but an image ofan object such as a car driving through the town or a Fouriertransformed image irradiated is also available. It is obvious that anyimages which are irradiated for at least a specific period and havewavelengths which are adequate to generate carriers in thephotoconductive layer 105 are available.

Though the auxiliary light source 13 is provided separately in the aboveembodiment, the read out light 5 itself can be used in place of theauxiliary light source 13 when the FLC-OASLM 1 does not have thedielectric mirror 17 and the light blocking layer used as the lightreflecting and separating layer, or when the read out light 5 affectsthe photoconductive layer 105, for instance, the intensity of the readout light is too strong, even if the FLC-OASLM 1 has the lightreflecting and separating layer.

FIGS. 7(a)-7(c) illustrate a driving method of another embodiment of asystem and method for adjusting the sensitivity of the inventiveoptically addressed spatial light modulator. FIG. 7(a) shows a drivingvoltage wave form of the FLC-OASLM 1 in case of grounding thetransparent electrode layer 102a of the read side, FIG. 7(b) shows achange in intensity of the read out light, and FIG. 7(c) shows a changein intensity of an image which is read out from the FLC-OASLM 1. Thisembodiment is different from the embodiment shown in FIGS. 6(a)-6(c) inthat since the read out light 5 acts on the write voltage applicationtime as well as the bias light 6 disclosed in the above-mentionedembodiment by overlapping periods of the write pulse 32 and read outlight irradiation 34 for a certain time T0, the threshold of the writelight can be decreased. Further, a change in the threshold of the writelight can be easily controlled by changing a value of T0. However, as inthis case the read out light generally irradiates uniform light, it isimpossible to distribute the sensitivity within a plane of the read sideby distributing intensity of the read out light 5. In this case, lightintensity of an image which is read out as a positive image is brightdue to the irradiation of the read out light 5, and is turned to darkwhen the erase pulse 31 is applied.

FIGS. 8(a)-8(c) illustrate a driving method of another embodiment andshow a method for adjusting the sensitivity of the inventive opticallyaddressed spatial light modulator. FIG. 8(a) shows a driving voltagewaveform of the FLC-OASLM 1 while grounding the transparent electrodelayer 102a of the read side. FIG. 8(b) shows a change in intensity ofthe read out light. FIG. 8(c) shows a change in intensity of an imagewhich is read out from the FLC-OASLM 1. In this driving method, when theintensity of the read out light 5 is modulated, the read out light 5irradiates the read side 3 during the write voltage application time,and the light is used as a bias light 35. If a laser diode is used as alight source of the read out light 5, this method can be easily embodiedby changing the driving voltage applied to the laser diode according tothe intensity of the read out light 5. In other words, the intensity ofthe bias light 35 can be changed by adjusting the driving voltage of thelaser diode, by providing a liquid crystal shutter in the optical pathof the read out light 5, by rotating a polarization plate which isarranged in front of or behind the liquid crystal shutter or rotatingthe liquid crystal shutter itself, and adjusting the contrast of theread out light 5 being transmitted through the liquid crystal shutter.However, the intensity of the read out light irradiation 34 alsochanges.

As explained above, in the method for adjusting the sensitivity of theinventive optically addressed spatial light modulator, especially incase of the FLC-OASLM, the sensitivity of the FLC-OASLM can be easilycontrolled by irradiating the bias light during the write voltageapplication time of the FLC-OASLM, and changing the intensity and theirradiation time of the bias light. As a result, it is possible torecord a written image the intensity of which is either entirely orpartially weak, as well as a part of a written image with weakintensity. Further, as the threshold of the write light of the FLC-OASLMcan be easily changed, it is also possible to obtain an image of whichthe threshold for binary recording is changed.

As a result of the above, the application scope of the FLC-OASLM can beexpanded, such as the use in an incoherent to coherent transducer. Inaddition, when the FLC-OASLM is used for pattern recognition and thelike, pattern recognition can be performed with higher accuracy, forinstance, and it becomes possible to record the higher frequencycomponent of a Fourier transformed image. Further, since means forirradiating the bias light does not additionally include any expensiveor complicated items, there is an advantage in that it is notaccompanied with a significant rise in cost.

What is claimed is:
 1. An optically addressed spatial light modulationsystem comprising: a spatial light modulator; writing light irradiationmeans for irradiating a writing light to record an image onto thespatial light modulator; read-out light irradiation means forirradiating a bias light onto the spatial light modulator to adjust arecording sensitivity of the spatial light modulator and for irradiatinga read-out light to read out an optical image from the spatial lightmodulator; and bias light adjusting means for adjusting the bias lightto adjust the sensitivity of the spatial light modulator.
 2. Anoptically addressed spatial light modulation system according to claim1; wherein the bias light adjusting means includes means for adjustingat least one of the irradiation time and the light intensity of the biaslight.
 3. An optically addressed spatial light modulation systemaccording to claim 1; further comprising driving means for supplyingvoltage signals to the spatial light modulator.
 4. An opticallyaddressed spatial light modulation system according to claim 3; whereinthe driving means includes means for supplying voltage signals whichinclude writing voltage signals; and the bias light adjusting meansincludes means for controlling synchronous signals applied to both thedriving means and the read-out light irradiating means to adjust anoverlapping time period of application of the bias light and applicationof a writing voltage signal.
 5. An optically addressed spatial lightmodulation system according to claim 3; wherein the driving meansincludes means for supplying voltage signals which include writingvoltage signals for recording an image to the spatial light modulatorduring irradiation with a writing light, erasing voltage signals forerasing an image recorded on the spatial light modulator, and read-outvoltage signals supplied to the spatial light modulator duringirradiation with a read-out light for reading out an image previouslyrecorded on the spatial light modulator.
 6. An optically addressedspatial light modulation system according to claim 3; wherein thewriting light irradiation means comprises a CRT and an imaging lens; theread-out light irradiation means comprises a read-out laser, a powersource for powering the read-out laser, a beam expander for expanding alaser beam emitted by the read-out laser, a polarization beam splitterfor reflecting the read-out light onto a read-out side of the spatiallight modulator, a diffusion plate for eliminating an intensitydistribution of the bias light, and a projection lens for projecting thebias light; and the bias light adjusting means includes means foradjusting one of the intensity and the irradiation time of the biaslight and a circuit for supplying synchronizing signals forsynchronizing the driving means and the power source of the read-outlaser.
 7. An optically addressed spatial light modulation systemaccording to claim 1; wherein the spatial light modulator comprises apair of opposing transparent substrates sandwiching a light modulationmaterial layer, and a photoconductive layer formed on at least one innerside of the pair of transparent substrates.
 8. An optically addressedspatial light modulation system according to claim 7; further comprisingdriving means for supplying voltage signals including writing voltagesignals, read-out voltage signals and erasing voltage signals to thespatial light modulator.
 9. An optically addressed spatial lightmodulation system according to claim 8; further comprising control meansfor controlling the driving means such that no read-out voltage signalsare supplied to the spatial light modulator when writing voltage signalsare applied thereto.
 10. An optically addressed spatial light modulationsystem according to claim 7; wherein the light modulation material layercomprises a liquid crystal layer.
 11. An optically addressed spatiallight modulation system according to claim 10; wherein the liquidcrystal layer is composed of a ferroelectric liquid crystal material.12. An optically addressed spatial light modulation system according toclaim 7; wherein the light modulation material layer comprises a BSOcrystal material layer.
 13. An optically addressed spatial lightmodulation system according to claim 7; wherein the light modulationmaterial layer comprises a lithium niobate crystal material layer. 14.An optically addressed spatial light modulation system according toclaim 1; wherein the spatial light modulator comprises a pair oftransparent substrates sandwiching a bistable ferroelectric liquidcrystal layer, and a photoconductive layer formed of amorphous siliconon at least one inner side of the pair of transparent substrates.
 15. Anoptically addressed spatial light modulation system according to claim1; wherein the bias light adjusting means includes means for adjustingat least one of the irradiation time and the light intensity of the biaslight.
 16. An optically addressed spatial light modulation systemaccording to claim 1; wherein the read-out light irradiation meansincludes means for irradiating a bias light having a uniform lightintensity distribution.
 17. A method for driving an optically addressedspatial light modulation system having a spatial light modulator,writing light irradiation means for irradiating a writing light, andread-out light irradiation means comprising a power source and a lightsource for irradiating a bias light and a read-out light, the methodcomprising the steps of: irradiating a writing light from the writinglight irradiation means onto the spatial light modulator to record animage; irradiating a bias light from the read-out light irradiationmeans onto the spatial light modulator to adjust a recording sensitivityof the spatial light modulator while irradiating the writing light ontothe spatial light modulator; irradiating a read-out light from theread-out light irradiation means onto the spatial light modulator; andcontrolling the electrical power from the power source to adjust one ofthe light intensity and the illumination time of the bias light.
 18. Amethod for driving an optically addressed spatial light modulationsystem according to claim 17; further comprising the step of applying anerasing voltage signal to the spatial light modulator to erase anoptical image.
 19. A method for driving an optically addressed spatiallight modulation system according to claim 18; further comprising thesteps of applying a writing voltage signal to the spatial lightmodulator during a time period when a writing light is irradiated, andapplying a read-out voltage signal to the spatial light modulator duringa time period when a read-out light is illuminated.
 20. A method fordriving an optically addressed spatial light modulation system having aspatial light modulator, writing light irradiation means for irradiatinga writing light, read-out light irradiation means comprising a powersource and a light source for irradiating a bias light and a read-outlight, driving means for supplying voltage signals including writingvoltage signals and read out voltage signals to the spatial lightmodulator, and bias light adjusting means for adjusting at least one ofthe light intensity and the illumination time period of the bias light,the method comprising the steps of: irradiating a writing light from thewriting light irradiation means onto the spatial light modulator torecord an image; irradiating a bias light from the read-out lightirradiating means onto the spatial light modulator while irradiating thewriting light to adjust a recording sensitivity of the spatial lightmodulator; irradiating a read-out light from the read-out lightirradiation means onto the spatial light modulator to read out arecorded image; and controlling synchronous signals applied to both thedriving means and the power source to control a time period during whichthe bias light is irradiated and a writing voltage signal is beingapplied to the spatial light modulator.
 21. A method for driving anoptically addressed spatial light modulation system according to claim20; further comprising the step of applying an erasing voltage signal tothe spatial light modulator to erase an optical image.